Even in a homogeneous conditions era like the current one (where it is easier for the top top players, in my opinion, to inflate their nš of GS titles, finals, SF, QF,...), to win 10+ GS is very very difficult.

In about 34 years (from 1968 to about 2001, both included) of polarized conditions eras, only two players did it (Borg with 11 and Sampras with 14 ).

In about a decade of homogenous conditions era, another two players have done it (Federer with 17 and Nadal with 11 ).

Djokovic will probably get there, with 11 or 12 GS, but I think he is not winning 18 GS at all.

The only chance for him to achieve something like 15+ GS is Nadal not getting back to top-level and Murray stopping improving so that he could be head and shoulders the best player for the following 3 or 4 years (something similar to Federer 2004-2007 seasons).